Has Gained or Had Gained?

I have gained means you are referring to the experience you gained until today. I gained is referring to the experience you gained in the past. Hardly somebody would think you didn't gain any experience, recently, if you say I gained.

Has gained meaning?

B1. to get something that is useful, that gives you an advantage, or that is in some way positive, especially over a period of time: The mayor has gained a lot of support from the teacher's union.

Has gained in a sentence?

Gained sentence example. Nothing could be gained by dwelling on such thoughts. Nothing could be gained by remaining still. I've gained five pounds.